I. Field of the Invention
The present invention relates to a vehicle rudder angle control apparatus having an active function and, more particularly, to a fail-safe unit incorporated in a steering system to prevent overrunning of an actuator during failure of a control system except for a power assisting unit to prevent a power-lock phenomenon, and effectively to utilize a power assisting function.
II. Description of the Related Art
High-performance vehicles have been developed along with the recent advancement of electronic techniques. Strong demand has arisen for improving vehicle driving stability in order to decrease fatigue during high-speed, long-distance driving and assure safety at the time of rapid steering.
For example, in a strong side wind, a driver must always concentrate on driving only which leads to increase in mental fatigue. In particular, when a vehicle on a crowded urban highway receives a strong side wind from a valley of buildings, the vehicle encounters a very dangerous phenomenon. In addition, driving at nighttime narrows the field of view and the reaction driver becomes slow in steering action, thus greatly increasing a possibility of danger.
In order to solve the above problems, appropriate vehicle traveling control is demanded wherein motion of the vehicle is detected by various types of sensors and vehicle motion information detected by the sensors is processed by a computer. Therefore, a computer-aided steering support system is required to positively steer the rudder wheels so as to assist the driver's steering on the basis of computer control.
A steering apparatus for assisting driver's steering is proposed as a steering support system to satisfy the above needs. This steering apparatus comprises: an electronic control unit consisting of an electronic circuit; and a servo actuator for performing a power assisting operation. The actuator receives electrical, hydraulic (oil), or pneumatic (air) energy.
In this system, however, when the electronic control unit the failed, "overrunning" or the "power-lock phenomenon" of the actuator occurs to cause a very dangerous condition. The "power-lock phenomenon" is one occurring in a mechanical feedback tracking servo actuator system shown in FIG. 6. A driver cannot steer because the actuator operates in a direction to prevent his steering. More specifically, assume that the electronic control unit or an energy converting element (e.g., an electric motor) for controlling the actuator in accordance with a signal from the electronic control unit does not properly function. In this case, even if the driver turns the steering wheel, i.e., even if a steering force is mechanically applied from a steering shaft to a pinion shaft through a coupling having a predetermined play, an electrohydraulic or pneumatic power is supplied in response to slight movement (rotational angle or displacement) of the pinion shaft or the actuator rod. Further steering cannot be performed. Therefore, the power force acts a direction opposite the direction of the steering force. In an actuator having a small power acting amount, steering will not be completely impossible, but it is very difficult to turn the steering wheel.
In order to solve the above problem of the conventional steering support system, there is proposed a "vehicle steering apparatus" (Japanese Laid-Open Utility Model Publication No. 61-35082) in which a fail-safe unit comprising a solenoid-valve is built into a device for driving a rudder mechanism of rudder wheels. As shown in FIG. 6, this steering apparatus includes a clutch means CH1 connected between an input shaft IA connected to a steering wheel SW and an output shaft OA connected to a rudder control mechanism of each steerable or rudder wheel of the vehicle. The clutch means CH1 is switched from the "OFF" state to the "ON" state in an abnormal state of an output shaft drive unit DD. A clutch means CH2 which performs an engaging/disengaging operation in response to an output signal of a control unit CR for controlling an operating condition of the drive unit DD is arranged between the output shaft OA and the output shaft drive unit DD. When the clutch means CH1 arranged between the input and output shafts IA and OA is switched from the "OFF" state to the "ON" state, a control signal for switching the clutch means CH2 from the "ON" state to the "OFF" state is generated by the control unit. A manual steering force can be smoothly transmitted, and the "steering wheel locking phenomenon" can be prevented. In addition, a conventional drawback which acts an excessive load on the steering wheel to degrade operability can be prevented.
In this conventional steering apparatus, however, the fail-safe unit is operated on the basis of a command from the electronic control unit. When the electrical system fails, the apparatus malfunctions to disable steering. Even if the electrical system is normally operated, one chamber of a power cylinder simply communicates with the other chamber, and an increase in driver's steering force is required. In particular, a very large steering force is required when steering is performed at a gear ratio under the assumption of power assistance. Without power assistance is difficult for the driver to steer.
As described above, proper functioning indispensable to the fail-safe unit cannot be assured in the conventional apparatus.
The present inventors have made extensive studies to solve the conventional problems described above and various systematic experiments and have achieved the present invention.
The present inventors have solved the major conventional problems not by employing electrical system control having a higher possibility of danger in the fail-safe unit upon occurrence of an abnormal state of a vehicle rudder angle control apparatus, but by employing an arrangement utilizing a mechanical means having a higher possibility of assurance and a power medium.